In the manufacture of electrical/mechanical devices wherein a mechanism is disposed in a housing and potted with a cured mass of a resinous composition to provide a barrier between the mechanism and the external environment of the device, a number of curable compositions have been employed.
For example, Japanese patent application No. 57/134,923 discloses the manufacture of an electrolytic capacitor in which the capacitor element is positioned in an aluminum case and sealed with rubber prior to potting of the assembly with an epoxy or phenolic resin.
West German Offenlegungsschrift No. 2,649,070 teaches the encapsulation of polystyrene capacitors with a thermosetting epoxy in a mold at elevated temperature.
Thermosetting epoxy resins have come into broad usage in such potting applications due to their favorable properties, including high strength, hardness, solvent resistance, good adhesion to conventionally employed housing materials, thermal stability, gas impermeability, and ready curability by a variety of mechanisms and numerous curatives over a wide range of elevated temperature cure conditions.
Despite their significant advantages, however, thermosetting epoxy resins have associated disadvantages which have limited their use, or otherwise rendered their employment in the aforementioned potting applications difficult in the context of commercial high volume manufacturing operations.
A primary disadvantage of thermosetting epoxy resins in the potting of electrical/mechanical assemblies is that their viscosities decrease with increasing temperature under curing conditions until the onset of gellation is reached.
Thus, as the resinous mass dispensed into the housing of the device is subjected to elevated temperature, e.g., on the order of 150-400 degrees F., as in a cure oven, the viscosity of the epoxy resin progressively decreases. The positional stability of the resinous mass then is lost and it migrates or "runs" within the housing.
Such migratory behavior is deleterious due to its tendency to foul the mechanism in the housing. When the mechanism is mechanical or electromechanical in character, and thus includes moving parts, the fouling may interfere with or prevent the proper operation of the mechanism, rendering it useless for its intended purpose. When the mechanism is electrical in character, the fouling may change the conductivity, capacitance, resistance or other critical characteristics of the mechanism's components, ruining the device and requiring its rejection in the manufacturing operation.
Another significant disadvantage of the use of thermosetting epoxy resins in the potting of electrical/mechanical devices is associated with air or other ambient gas which is trapped around the mechanism when the resin is dispensed into the housing, particularly when the mechanism contains voids from which the ambient gas is not readily displaced when the liquid resin is introduced.
The trapped gas in the housing during the curing of the epoxy resin then may bubble through the curing mass, particularly since its viscosity is reduced at this stage, and produce gas channeling void formation in the finally cured composition mass. Frequently, this gas bubbling effect is extensive enough to create continuous leak path channels in the finally cured mass, with the result that the intended barrier function of the cured mass is defeated.
The gas bubbling effect is also present when ambient cure epoxy resins are employed, although to a lesser extent.
In some electrical/mechanical devices, the mechanism is interiorly disposed in the housing and the housing is provided with an opening through which a structural element is inserted and joined to the interior mechanism after the latter has been potted and the resinous mass fully cured. An example is a trimming potentiometer, wherein the lead screw or gear shaft is inserted into the housing to engage a wiper assembly whereby input voltage to the device may be divided as a function of the mechanical input.
In such devices the housing opening usefully serves as a "blow hole" accomodating the release of gas during he curing of the potting mass. The opening, however, also provides a flow path which may aid the migratory fouling of the interior machanism as well migratory occlusion of the opening itself.
Relative to the resin compositions of the present invention which overcome the aforementioned problems, relevant art includes U.S. Pat. No. 4,412,048; Australian Patent Application No. 53,233/79; U.S. Pat. Nos. 4,479,990; 4,239,077; 4,025,407; 4,552,604; 4,427,478; European Patent Application No. 99,856; and Japanese Patent Application No. 60/84,715, disclosing acrylic/epoxy resin compositions which are sequentially UV and thermally cured. Other related art includes U.S. Pat. No. 4,444,806 (co-heat cured acrylic/epoxy compositions); U.S. Pat. No. 4,548,895 (composition containing 1,2-epoxy compounds and acrylates which are first thermal cured, then photocured by actinic radiation exposure); and U.S. Pat. Nos. 4,288,527 and 4,352,723 (pinacol-containing acrylate compositions optionally with a copolymerizable monomer or reactive diluent, which "are cured by both ultraviolet radiation and heat, in either order or simultaneously to form solid products suitable for use as . . . sealants").